Thematic ArticlePolyphase accretionary tectonics in the Jurassic to Cretaceous

accretionary belts of central Japan

OSAMU TAKAHASHIDepartment of Astronomy and Earth Sciences, Tokyo Gakugei University, Koganei, Tokyo 184-8501, Japan,

(E-mail: takahasi@u-gakugei.ac.jp)

Abstract Mesozoic accretionary complexes of the southern Chichibu and the northern Shimanto Belts, widely exposed in the Kanto Mountains, consist of 15 tectonostratigraphicunits according to radiolarian biochronologic data. The units show a zonal arrangement ofimbricate structure and the age of the terrigenous clastics of each unit indicates succes-sive and systematic southwestward younging. Although rocks in these complexes range inage from Carboniferous to Cretaceous, the trench-fill deposits corresponding to the Hau-terivian, the Aptian to Middle Albian and the Turonian are missing. A close relationshipbetween the missing accretionary complexes and the development of strike-slip basins isrecognizable. The tectonic nature of the continental margin might have resulted from achange from a convergent into a transform or oblique-slip condition, so that strike-slipbasins were formed along the mobile zones on the ancient accretionary complexes. Mostterrigenous materials were probably trapped by the strike-slip basins. Then, the accretionof the clastic rock sequence occurred, probably as a result of the small supply of terri-genous materials in the trench. However, in the case of right-angle subduction, terrige-nous materials might have been transported to the trench through submarine canyons anddeposited there. Thus, the accretionary complexes grew rapidly and thickened. Changesboth in oceanic plate motion and in the fluctuation of terrigenous supply due to the sedi-mentary trap caused pulses of accretionary complex growth during Jurassic and Creta-ceous times. In the Kanto Mountains, three tectonic phases are recognized, reflecting thechanges of the consuming direction of the oceanic plates along the eastern margin of theAsian continent. These are the Early Jurassic to early Early Cretaceous right-angle subduction of the Izanagi Plate, the Early to early Late Cretaceous strike-slip movementof the Izanagi and Kula Plates, and the late Late Cretaceous right-angle subduction of theKula Plate.

Key words: accretion, Cretaceous, Izanagi Plate, Jurassic, Kula Plate, oceanic plate stratigraphy, radiolarian biostratigraphy, strike-slip basin.

accretionary complexes achieved in recent yearshave revealed the structure, stratigraphy and tec-tonic evolution of the Jurassic to Tertiary in theJapanese Islands, and continuous accretionaryprocesses were assumed for them (Matsuoka 1984;Nakae 1993; Nishizono 1996). These studies haveclarified that the radiolarian biostratigraphy andits chronology are very important and useful toolsto decipher accretionary tectonics.

In the Kanto Mountains, central Japan, LateMesozoic accretionary belts, such as the Samba-gawa, Chichibu, and Shimanto Belts, are widely

INTRODUCTION

The increase in the biochronologic data based onradiolarian fossils has created a better under-standing of the evolution of the Chichibu and Shimanto Belts of South west Japan in terms ofsubduction–accretion processes (Taira et al. 1980;Matsuoka 1984; Taira et al. 1988; Matsuda &Isozaki 1991). The detailed mapping and dating of

Accepted for publication 13 April 1999.©1999 Blackwell Science Asia Pty Ltd.

The Island Arc (1999) 8, 349–358

350 O. Takahashi

distributed from north to south (Fig. 1). Thedetailed analysis of radiolarian fossils and litho-facies of the southern part of the Chichibu Belt and the northern part of the Shimanto Belt in the Kanto Mountains has revealed that theaccretionary complexes are divided into 15tectonostratigraphic units (Takahashi 1995). Thesetectonostratigraphic units were not constantlybeing accreted in those geologic times; the accre-tion occurred in phases.

This paper more clearly defines the episodicgrowths of the Late Mesozoic accretionary belts ofthe Outer Zone of South west Japan, based ondetailed radiolarian zoning of the Kanto Moun-tains (Takahashi & Ishii 1995) and discusses thetectonic factors controlling the formation of theaccretionary complexes.

GEOLOGIC SETTING OF THE JURASSIC–CRETACEOUS STRATA IN THE KANTO MOUNTAINS

The Kanto Mountains, located approximately 50 km northwest of Tokyo, are geotectonicallydivided into the Sambagawa, Chichibu and Shimanto Belts from north to south. Most of therocks in the Sambagawa and Chichibu Belts havebeen assigned to the Jurassic to Early Cretaceousaccretionary complexes (Sashida et al. 1982; Ozawa& Kobayashi 1985; Ishii & Takahashi 1993), andthose in the Shimanto Belt to the Late Cretaceous

and Paleogene accretionary complexes (Sakai1987; Takahashi et al. 1989; Iyota et al. 1994), whichshow different styles of subduction–accretionprocesses. In addition, less deformed, shallow-marine Cretaceous sequences are distributed intwo narrow belts of the Sanchu graben in the southand the Atokura-Shoya belts in the north. They arein fault contact with the adjacent accretionary com-plexes. This zonal arrangement of the geologicunits in the Outer Zone of Southwest Japan istraceable for over 2000 km from the Kanto Moun-tains to Kyushu (Fig. 2).

TECTONOSTRATIGRAPHIC FEATURES OF THEJURASSIC–CRETACEOUS ACCRETIONARYCOMPLEXES

Accretionary complexes of the southern part of theChichibu and the northern part of the ShimantoBelts of the Kanto Mountains are characterized byan assemblage of 15 thrust sheets. Each of thethrust sheets has been defined as an independenttectonostratigraphic unit and numbered fromnorth to south: 1–15 (Takahashi 1995) (Fig.3). Although all the units in the investigated area dip northeastward, the ages of the terri-genous clastics (mainly mudstone) of each unit indicate successive and systematic southwestward younging (Table 1).

The tectonostratigraphic units of the Chichibu

Fig. 1 Index map of Southwest Japanshowing the distribution of the Sambagawa,Chichibu and Shimanto Belts, together with fault-controlled Cretaceous basins.The study area is indicated by rectangles.(a) Goshonoura and Himenoura; (b) Umi-noura, Yatsushiro, Mifune and Kumamoto;(c) Torinosu; (d) Yamabu and Haidateyama;(e) Onogawa; (f) Torinosu; (g) Monobe andRyoseki; (h) Soto-izumi; (i) Doganaro; (j)Izumi; (k) Futagawa; (l) Yuasa; (m) Idaira,Todai and Misakubo; (n) Sanchu; (o)Atokura and Shoya; P, Choshi. [BTL,Butsuzo Tectonic Line; MTL, Median Tectonic Line.]

Polyphase accretionary tectonics in central Japan 351

Fig. 2 Geologic sketch map showingthe tectonic divisions of the KantoMountains (adapted from Takahashi andIshii 1995). 1, Sanchu Graben; 2,Atokura Basin; 3, Shoya Basin. [BTL,Butsuzo Tectonic Line; MTL, MedianTectonic Line.]

Fig. 3 Tectonostratigraphic map of thesouthern Chichibu and northern ShimantoBelts of the Kanto Mountains showing the15 tectonostratigraphic units of accre-tionary complexes which, numbered fromnorth to south, are 1–15. The ages of theterrigenous clastics of each unit indicatesuccessive southwestward younging.

352 O. Takahashi

and Shimanto Belts are classified into two types of lithofacies: coherent-type and melange-type;they are different in their deformation and lithofacies. In the Chichibu Belt, the chert-clasticrock sequence is assigned to the coherent-type (Units 4–8). This rock sequence shows, pri-marily, a stratigraphic succession from chert to siliceous mudstone, mudstone and sandstone, in ascending order. Greenstone and limestone frequently accompany chert at the basal part.They range in age from Permian to early LateJurassic (Ishii 1962; Takashima & Koike 1984;Hisada & Kishida 1986; Sakai 1987; Sashida 1988;Takahashi & Ishii 1995). Similar stratigraphic successions have been reported by many research-ers from Mesozoic accretionary complexes inSouth-west Japan (Taira et al. 1980; Matsuoka1984; Wakita 1988; Matsuda & Isozaki 1991; Nakae1993).

The melange-type units in the Chichibu Belt(Units 1–3 and 9–11) include blocks of greenstone,limestone, chert and sandstone in various shapesand sizes within a phyllitic muddy matrix. Theyshow a block-in-matrix fabric but their originalsuccession is similar to the chert-clastic rock se-quence mentioned above. According to Ozawa andKobayashi (1985) and Sakai (1987), fossils such asfusulinacea, conodonts, radiolarians and molluscsobtained from these blocks show a wide range ofgeologic age from Carboniferous to Late Jurassic.In contrast, the mudstone matrix contains EarlyJurassic to Early Cretaceous radiolarian fossils(Takahashi & Ishii 1995).

The northern part of the Shimanto Belt is com-

posed mainly of alternating beds of sandstone and mudstone (Units 12, 14, and 15) and containsradiolarian fossils of the Early to Late Cretaceous(Takahashi & Ishii 1995). The melange-type unit(Unit 13) includes a heterogeneous assemblage ofblocks of greenstone, limestone, chert and sand-stone ranging in age from the Triassic to the earlyLate Cretaceous (Takashima & Koike 1984;Sashida et al. 1989; Iyota et al. 1994; Takahashi &Ishii 1995). Radiolarians from the mudstonematrix of melanges indicate Late Cretaceous(Takahashi & Ishii 1995).

The boundary between the Chichibu and Shi-manto Belts is marked by a northwest-strikingthrust, the Butsuzo Tectonic Line (BTL). The dif-ference of the lithofacies in the Chichibu Belt fromthose in the Shimanto Belt is the greater abun-dance of the pelagic–hemipelagic deposits such aslimestone and chert, whereas the complexes of the Shimanto Belt consist mainly of monotonousflysch successions of sandstone and mudstone but with less abundant greenstone and chert andless limestone. It is widely accepted that the BTLis the important thrust throughout SouthwestJapan.

FAULT-CONTROLLED CRETACEOUS BASINS

Fault-controlled Cretaceous basins (the SanchuGraben and the Atokura and Shoya Basins) occuras narrow zones in fault contact with the olderaccretionary complexes.

The Sanchu Graben strata exposed in the

Table 1 Geologic age of trench-fill deposits of tectonostratigraphic units in the Kanto Mountains. Units 1–15 correspond tothose in Figs 3 and 4. Radiolarian zones and ages are after Takahashi and Ishii (1995) and Matsuoka (1995)

Unit Radiolarian assemblage zone of Radiolarian interval zone of AgeTakahashi and Ishii (1995) Matsuoka (1995)

Unit 15 Ae–Cg — Early Campanian–MaastrichtianUnit 14 Ae–At — Early Campanian–Early MaastrichtianUnit 13 Hb–Ae — Late Albian–Middle CampanianUnit 12 Hb–Df — Late Albian–SantonianUnit 11 Tc — BarremianUnit 10 Pp–Su JR8–KR2 Tithonian–ValanginianUnit 9 Ty JR7 KimmeridgianUnit 8 Ss JR6 Late Callovian–OxfordianUnit 7 Mg JR5 Middle CallovianUnit 6 Mg JR5 Middle CallovianUnit 5 Mg JR5 Middle CallovianUnit 4 Gn JR5 Early CallovianUnit 3 Ue–Dk JR4–JR5 Bajocian–BathonianUnit 2 Hh–Dk JR3–JR5 Aalenian–BathonianUnit 1 Ps–Pg JR1–JR2 Hettangian–Toarcian

Chichibu Belt consist of non-marine and marineclastic sequences about 2700 m thick. The age, assuggested by ammonoids, indicates Hauterivian toAlbian (Matsukawa 1983). Both the northern andsouthern margins of the Sanchu graben are inhigh-angle fault contact with the Jurassic accre-tionary complexes. The northern boundary fault isregarded as a right-slip fault (Sekiyama et al.1984), and the southern margin as a left-slip fault(Hisada et al. 1987).

The Atokura Basin, situated just at the south-ern margin of the Sambagawa Metamorphic Belt,is filled by the Atokura Formation composed ofconglomerates and alternating beds of sandstoneand mudstone. It shows a tightly closed synclinalstructure with steep, northward-dipping axialplanes. The axis is parallel to the general trend ofthe formation. Ammonoids and molluscs denote,possibly, Turonian to Santonian ages (Arai et al.1963; Watanabe et al. 1990). The boundariesbetween the Atokura Basin, the Sambagawa Beltand the Chichibu Belt show steeply dipping faultcontacts. However, the formation has been inter-preted as showing a nappe structure because ofthe existence of gently dipping thrusts at thewestern part of the distribution (Fujimoto et al.1953; Takagi & Fujimori 1989).

The Shoya Basin located at the northwesternmargin of the Kanto Mountains is represented bythe Shoya Formation, which consists of a marinesequence about 500 m thick with a fining-upwardcycle. Takahashi & Ishii (1993) reported Late Cre-taceous (Late Campanian to Early Maastrichtian)radiolarian fossils from the upper part of the for-mation. The Shoya Formation was deformed intothe east–west trending syncline plunging gently tothe west. The southern margin of the formation isunconformably overlain by Tertiary deposits andits northern margin is in fault contact with them.It is thought that this fault has moved vertically at least twice (Watanabe 1954). However, theoccurrence of porphyroclasts in the fracture zonesuggests a possibility of the left-slip movement ofthe fault (Takahashi et al. 1992).

MISSING TRENCH-FILL DEPOSITS IN THEJURASSIC–CRETACEOUS ACCRETIONARYCOMPLEXES

As described above, tectonostratigraphic units inthe Chichibu and Shimanto Belts in the study areahad primarily formed stratigraphic successionsfrom greenstone or limestone to chert, siliceous

mudstone, mudstone and sandstone, in ascendingorder. Some units often lack a part or most of theseconstituents but the basic combination is consis-tent. For example, the melanges are extremely disrupted lithologic units with a wide range of radiolarian zones resulting from the promiscuousmixture of various stratigraphic units, though insome cases, original stratigraphic successionscould be reconstructed by the analysis of radio-larian fossils and lithofacies (Takahashi 1995).

This basic lithologic succession could be definedas the ‘oceanic plate stratigraphy’, which beginswith pelagic sediments (chert and limestone),through hemipelagic sediments (siliceous mud-stone) and ends with terrigenous deposits (mud-stone and sandstone) (Taira et al. 1980; Isozaki etal. 1990). This suite of sedimentary successionrecords a history of sedimentation on the oceanfloor from the mid-oceanic ridge to the trenchduring a period of plate motion (Taira et al. 1980;Matsuda & Isozaki 1991). The approximate timingof the arrival of a plate at the trench can be determined by the age of the lithologic boundarybetween the hemipelagic and overlying terri-genous deposits; moreover, the time-span of terri-genous deposits indicates a duration of trench-fill sedimentation. The youngest age of the accre-tionary units may indicate the terminal time of the terrigenous sedimentation at the subductionzone.

Figure 4 shows the missing trench-fill depositsduring the Hauterivian, Aptian to Middle Albianand Turonian in the Kanto Mountains. This sug-gests little or almost no accretion of the clastic-rock sequence occurred in the trench (probablybecause of the limited supply of terrigenous clas-tics) which indicates a change in the direction ofmovement of oceanic plates or perhaps tectonicerosion.

DEVELOPMENT OF FAULT-CONTROLLED BASINSSYNCHRONIZED WITH MID-CRETACEOUSACCRETIONARY HIATUS

Fault-controlled Cretaceous basins seem toprovide a key to resolving accretionary hiatusevents. The depositional ages of the Sanchugraben strata, the Atokura Formation and theShoya Formation range from Hauterivian toAlbian, Turonian to Santonian and Late Campan-ian to Early Maastrichtian, respectively. They areroughly synchronous with the events of the mid-Cretaceous accretionary hiatus. Figure 5 shows

Polyphase accretionary tectonics in central Japan 353

354 O. Takahashi

the range of the depositional ages of fault-controlled basins in the Outer Zone of SouthwestJapan. The deposition of the Early Cretaceousfault-controlled basins generally began at theHauterivian and ended at the Albian or Turonian,except for the Izumi, Himenoura, and TorinosuGroups. It is probable that a close temporal rela-tion existed between the missing trench-filldeposits and the development of fault-controlledbasins.

According to Engebretson et al. (1985), duringthe Early to early Late Cretaceous (ca. 135–85 Ma),the Izanagi Plate proceeded at high speed in anorth-northwest direction at the eastern margin ofthe Asian continent. The strike-slip tectonics may

have resulted from the strike-slip motion of theIzanagi Plate along the eastern margin of the Asian continent during the mid-Cretaceous. The Sanchu graben originated as a structural basin resulting from the strike-slip movement of the basement rocks (Sekiyama et al. 1984; Hisada et al. 1987). Taira et al. (1983) and Okada and Sakai (1993) also ascribed the fault-controlledbasins in Southwest Japan to strike-slip or oblique-slip faulting in the transform or oblique-slip plate boundary setting. Thus it is suggestedthat the tectonostratigraphic gaps in the KantoMountains occured under the same strike-slip tectonics as the development of the fault-controlled (strike-slip) basins along the eastern

Fig. 4 Age of the trench-fill depositscompiled from the 15 tectonostratigraphicunits in the Kanto Mountains. The numbers1–15 correspond to those in Fig. 3 andTable 1. Trench-fill deposits correlating tothe Hauterivian, Aptian to Middle Albianand Turonian are missing in the KantoMountains.

Fig. 5 Simplified stratigraphic chart ofJurassic to Cretaceous fault-controlledbasins in the Outer Zone of SouthwestJapan, (based on Matsumoto 1978 andMatsukawa and Obata 1992). Stipple (fill)indicates the periods of the missing trench-fill deposits in the Kanto Mountains. (a)Goshonoura and Himenoura; (b) Uminoura,Yatsushiro, Mifune and Kumamoto; (c)Torinosu; (d) Yamabu and Haidateyama; (e)Onogawa; (f) Torinosu; (g) Monobe andRyoseki; (h) Soto-izumi; (i) Doganaro; (j)Izumi; (k) Futagawa; (l) Yuasa; (m) Idaira,Todai and Misakubo; (n) Sanchu; (o)Atokura and Shoya; (p) Choshi.

margin of the Asian continent during the mid-Cretaceous.

Figure 6 shows a schematic relationship amongsedimentary basin, sedimentation, accretion, ter-rigenous materials and oceanic plate movementduring the mid-Cretaceous. The tectonic nature ofthe continental margin might have resulted from achange from a convergent into a transform oroblique-slip condition, so that strike-slip basinswere formed along the mobile zones on the ancientaccretionary complexes. Most terrigenous materi-als were probably trapped by the strike-slip basins(Fig. 6a). Then, little or almost no accretion of the clastic sequence occurred, probably because of the small supply of terrigenous materials in thetrench.

In contrast, in the case of right-angle subduc-tion, terrigenous materials might have been trans-ported to the trench through submarine canyonsand deposited at the trench (Fig. 6b). Thus, theaccretionary complex grew rapidly and thickened.Changes both in the oceanic plate motion and inthe fluctuation of terrigenous supply due to thesedimentary trap caused pulses of accretionarycomplex growth during the Jurassic and Creta-ceous in the Kanto Mountains.

Excellent documentation of the relationshipbetween accretionary complexes and deep-seaterrace deposits was given by the Scientific Party

(1980) and concerned the modern accretionarywedge of the Japan Trench. They showed an evolu-tion of the inner slope of the Japan Trench from theLate Oligocene to recent on the basis of the resultsof the Deep Sea Drilling Project (DSDP-IPOD)Legs 56 and 57. It showed the obvious process of thedisappearance of the deep-sea terrace and subse-quent appearance of accretionary complexes. Therelationship between accretionary complexes anddeep-sea terrace deposits and the behavior of terrigenous materials is quite similar in the JapanTrench accretionary prism and in the ancient subduction zone dealt with in this study.

The relationship between the fault-controlledbasins and the accretionary complex of the mid-Cretaceous in Kyushu was previously reported bySakai et al. (1990) and Sakai and Okada (1997).They inferred that the missing Shimanto accre-tionary complex was caused by the transformmovement or oblique-subduction of the Izangi–Kula Plates against the Eurasian Plate. Thus, it isof note that a close relationship between themissing accretionary complexes and the develop-ment of strike-slip basins is recognizable not onlyin the Kanto Mountains but also in Kyushu.

Progressive changes in the tectonic history ofthe southern part of the Chichibu Belt and thenorthern part of the Shimanto Belt in SouthwestJapan are inferred as follows. Three tectonicphases were recognized, reflecting the changes ofthe consuming direction of the oceanic plates; theEarly Jurassic to early Early Cretaceous right-angle subduction, the Early to early Late Creta-ceous transform or oblique-slip condition causingthe oblique- or strike-slip tectonics along the mid-Cretaceous eastern margin of the Asian continentand the late Late Cretaceous right-angle sub-duction. According to the data of Isozaki andMaruyama (1991) and Isozaki (1996), these tectonicphases are thought to reflect the difference inmovement directions of the subducting plate. TheEarly Jurassic to early Early Cretaceous and thelate Late Cretaceous right-angle subductions arecorrelated to the subduction of the Izanagi andKula Plates, respectively.

CONCLUSION

This study has confirmed that little or no accretiontook place during the Hauterivian, Aptian toMiddle Albian and Turonian in the eastern part of the Chichibu and Shimanto Belts of SouthwestJapan. This phenomenon implies that the sub-

Polyphase accretionary tectonics in central Japan 355

Fig. 6 Schematic deposition–accretion model showing the relation-ship between the deposits of terrigenous materials and oceanic platemovement during the mid-Cretaceous. Thick arrows indicate the direc-tion of motion of the oceanic plate.

356 O. Takahashi

duction ceased at the continental margin duringthese periods owing to changes in the platearrangements.

This has been confirmed by Sakai and Okada(1997) in the Chichibu and Shimanto Belts inKyushu, the western part of Southwest Japan,though further studies are necessary to confirmthe same tectonic conditions in other parts of the East Asian continental margin. The resultsobtained will help in understanding the plume-related, very large igneous activity in the EarlyCretaceous at the East Asian continental margin,as noted by Okada (in press).

ACKNOWLEDGEMENTS

I am grateful to Drs Hakuyu Okada, JuichiYanagida and Tokihiko Matsuda, former Profes-sors of Geology at Kyushu University, and DrAtsushi Ishii and Associate Professor Masaki Mat-sukawa, Tokyo Gakugei University, for their valu-able discussion, advice and criticism. I am alsograteful to Dr Takashi Sakai, Kyushu University,and Dr Akira Sakai, Geologic Survey of Japan, andin particular to Dr Okada, Kyushu University, fortheir critical reading of drafts of the manuscript.Sincere thanks to Mr John Cantylon for his helpin revising the English.

This study was supported in part by the Grant-in-Aid for Co-operative Research (Grant No.06304001) and in part by the Grant-in-Aid forScientific Research (C) (Grant No. 07740398), bothfrom the Ministry of Education, Science, Sports,and Culture, Japan.

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